The invention relates generally to a circuit for energizing a permanent magnet starter motor for an internal combustion engine and more specifically to a circuit using pull-in and hold-in coils of a solenoid plunger to engage the pinion gear of a permanent magnet starter motor with the flywheel of an internal combustion engine.
Electric starter motor solenoid circuits using pull-in and hold-in coils are known which upon energization move a plunger driven by the coils until a pinion-type gear linked to the plunger meshes with the flywheel of the engine. One such prior art circuit is shown in FIG. 1 for example, wherein a solenoid 20 is connected between a power source and the starter motor 30. The power source includes a battery 14, a starter relay 12, and an ignition switch 10. The solenoid 20 comprises a set of plunger contacts 32 on the end of a conventional solenoid plunger 34 that is spring biased to place the contacts 32 in a normally open condition when the solenoid is deenergized. The movement of plunger 34 to close the plunger contacts 32 against the spring bias is provided by the magnetic field produced by pull-in coil 18 and a hold-in coil 16 when they are energized. The relay 12 contains a set of normally open contacts 13 and when energized closes the circuit so that both pull-in coil 18 and hold-in coil 16 of the solenoid become energized by the battery. When energized by the battery, the pull-in coil 18 causes plunger 34 of solenoid 20 to be drawn into position which closes solenoid contacts 32 and the starter motor 30 becomes connected to battery 14, thereby shunting pull-in coil 18 from the circuit. Hold-in coil 16 remains energized and has sufficient field strength to hold plunger contacts 32 closed until relay 12 is deenergized.
The pull-in coil 18 and hold-in coil 16 are made of insulated wound wire having an equal number of turns. This is conventionally important because upon deenergization of the circuit, the pull-in coil 18 and hold-in coil 16 are physically situated to magnetically cancel each other when relay 12 opens. The cancellation is needed because the permanent magnet starter motor 30 continues to spin after power is removed, thus becoming a generator while it is spinning down. Under the condition that the pull-in coil 18 is matched with hold-in coil 16, the following sequence occurs at the instant following the opening of ignition switch 10. Relay contacts 13 open, solenoid contacts 32 remain closed and battery 14 is still providing power to motor 30. The current flow through pull-in coil 18 is in the opposite direction of what it was when ignition switch 10 was initially closed while the direction of current flow through the hold-in coil. 16 is not changed. Because the magnetic field of the coils cancel each other, the solenoid plunger is magnetically released and the biasing spring (not shown) causes plunger contacts 32 to open and disconnect battery 14 from motor 30. The energy produced by the still-spinning motor 30 is dissipated through the hold-in coil 16 and the pull-in coil 18. Because the pull-in and hold-in coils are subject to a harsh environment, insulation on the coil windings can become degraded, causing shorts between the windings. Thus the effective number of windings in each coil are subject to change over time. If the number of turns in the coils are not matched, the magnetic field will not fully cancel and plunger contacts 32 may remain closed thereby leaving the starter motor 30 still connected to battery 14.
Normally the battery 14 and starter motor 30 are located at different locations in the engine compartment of the automobile and two power connections are provided to the solenoid 20 at terminals 22 and 24. A connection through the relay contacts 13 is provided at terminal 24 and a direct connection to battery 14 is provided at terminal 22 to one of the solenoid plunger contacts 32.
It would be desirable to continue to electrically isolate the battery from the starter motor and have only one battery connection to the solenoid.
It would also be desirable to eliminate the dependency of the circuit on the number of coils provided in the pull-in and hold-in coils.